Feature Request: plantuml support

[ystlong]

Hi, I would like to add plantuml support, but without luck it's not success.

I found markdown-it-plantuml plugin and add in marp mds_markdown.coffee

    plugins:
      'markdown-it-plantuml' : {
        openMarker: '```plantuml'
        closeMarker: '```'
        generateSource: (umlCode) ->
          return MdsMarkdown.plantumlServerUrl + "/" + plantuml_encoder.encode(umlCode)
      }
      'markdown-it-mark': {}
      'markdown-it-emoji':
        shortcuts: {}
      'markdown-it-katex': {}

MdsMarkdown.plantumlServerUrl is hard code in mds_markdown.coffee. it's work for render plantuml code block, such as:

a -> b

I want to set MdsMarkdown.plantumlServerUrl with global.marp.config, but it's not work.

My modify https://github.com/ystlong/marp

Would you consider add plantuml support?

Thank you!